1. Field of the Invention
The present invention relates to a recording sheet suitable for ink jet recording, and more specifically, to an ink jet recording sheet wherein the ink droplets which have been deposited on the surface of a paper are quickly absorbed into the sheet layer with suppression of the spread of an ink dot and a high recording density is hold.
2. Prior Art
The ink jet recording systems using ink jet elements generate almost no noise and are capable of high speed recording. Moreover, they can easily perform a multicolor recording with conventional plain paper. Owing to these features, these recording systems have been widely noted in recent years. And these systems have now been finding a wide range of application, including facsimile equipments, various printers and so on.
Generally, conventional plain paper can be used in the ink jet recording system. In general, the ink jet recording sheet for good recording must have the following two properties:
(1) An ink drop on the sheet surface is quickly absorbed into the sheet layer so that the paper becomes dry.
(2) The spread of an ink dot on the sheet surface is supressed.
The first property mentioned above is the most fundamental requirement in the ink jet recording sheet. In handling the recorded paper, if the ink remained on the sheet surface without being quickly absorbed into the sheet layer, the ink would cause stain. Especially, where a multichromatic recording is carried out, inks with two or more different colors are deposited at one point or very close to each other almost at a time, and consequently the ink amount per unit area on the sheet surface is increased in comparison with a monochromatic recording. Thus, good absorbency into the sheet layer is especially important.
The second property is a requirement for clear recording. It is essential that ink penetrates into the sheet so that the ink spreads into the under surface as well as on the surface of the sheet. On the other hand, optical density of the recorded sheet depends upon the ink amount per unit area of the sheet surface. Thus, optical density of the recorded sheet can be raised by suppressing both the spread of ink dots on the sheet surface and the penetration of ink droplets into the sheet layer.
Generally, the first and second requirements mentioned above are contradictory to each other because the sheet having great ink absorbency have large spreading speed of ink dots on the surface. Therefore, it is desirable to develop the ink jet recording sheet which satisfies the two requirements at the same time. In order to solve the above-mentioned problems, the inventors have proposed, in Japanese Patent Laid-Open No. 49113/1978, to use as an aqueous ink recording sheet, the sheet which was prepared by adding internally a large quantity of the fine powdered urea-formaldehyde resin into wood pulp and then coating water-soluble polymer on the surface. Although ink absorbency of the sheet which was prepared by adding a large amount of the fine powdered urea-formaldehyde resin was very high, its recording surface was a little white in a multi-color recording. That is, the optical density on the recording surface was a little low. Also, the color recording characteristic of this ink jet recording sheet was not always sufficient and besides, urea-formaldehyde resin was much more expensive than conventional fillers.